Accurate ECN support in tcpdump/libpcap

["Scheffenegger, Richard via tcpdump-workers" <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: "Scheffenegger, Richard" <Richard.Scheffenegger () netapp com>
>
> Date: Tue, 29 Aug 2023 14:33:28 +0000
>
> Hi,
>
> I’ve been asked to reach out to this mailing list, which, after much ado (temporarily unreactive mailman) I hope I can 
> finally do.
>
>
> The goal is to gather some feedback around the support for tcpdump to properly decode the upcoming (WGLC) Accurate ECN 
> signalling enhancement, which is part of the L4S (low loss, low latency, scalable) effort, ultimately culminating in a 
> variant of TCP called TCP Prague:
>
> Here is the change to tcpdump – adding the additional “A” TCP header flag bit; Since the ACK bit has traditionally been 
> mapped to “.”, and the refurbished former Nonce bit #9 in the 12-bit TCP header flags field has been named “AE” 
> (Accurate ecn Enable), the code changes to other tools such as Wireshark and Packetdrill are using the mapping to the 
> character “A” (or decode Accurate ECN stateful into the ACE counter value of 0..7)
>
> https://github.com/the-tcpdump-group/tcpdump/pull/999
>
>
> This change to the parser in libpcap allows access to all 12 bits when using the sample from the man page like this
>
> tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'
>
> to also include the ‘tcp-ae’ flag:
>
> https://github.com/the-tcpdump-group/libpcap/pull/1210
>
>
> If you are interested in learning more about L4S:
> https://datatracker.ietf.org/doc/rfc9330/
>
> Accurate ECN requirements:
> https://datatracker.ietf.org/doc/rfc7560/
>
> Accurate ECN signaling protocol:
> https://datatracker.ietf.org/doc/draft-ietf-tcpm-accurate-ecn
>
> The adjacent patch to packetdrill for decoding and encoding AccECN (stateful):
> https://github.com/google/packetdrill/commit/77ab15806a56de226345fbe1910139829e431fe0
>
> And the adjacent patch to wireshark – including the TCP Option of Accurate ECN
> https://gitlab.com/wireshark/wireshark/-/merge_requests/7816/diffs?commit_id=ecefcf880121961c03d2ec05ad378c82e951e0a2 
>
>
> Richard Scheffenegger 
>
>
> --- End Message ---
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